2 Answers

The default username and password is:
Username: admin
Password: admin
If this doesn't work then it most likely means that the password has been changed. In that case you should enter your own password but leave the username as admin.
And if you can't remember your password you may reset the modem/router back to factory default settings. Instructions for this can be found in the "These may also help" section further down on this page.
But If you use Windows operating system to connect the Internet (with PPTP, LT2P, or PPPoE), you can use the Dialupass utility to recover your ISP password. When you run this utility, it can instantly recover your password, as long as the password is stored by Windows.
If you use a router to connect the Internet, you can also try to use the AsterWin IE utility. This utility is quite old and was written many years ago in Visual Basic 6, but it still works with the latest version of Internet Explorer.